A work machine includes a vehicle body and a work implement attached to the vehicle body. A system calibrates the work machine by using an external measurement apparatus. The system includes an attitude sensor, a positional sensor attached to the vehicle body, a storage device, an input device and a processor. The attitude sensor outputs attitude data indicative of an attitude of the vehicle body. The storage device stores machine data indicative of a position of the positional sensor in a vehicle body coordinate system. The input device receives an input of calibration data including a position of a predetermined measurement point on the work machine measured by the external measurement apparatus, and a position of the positional sensor measured by the external measurement apparatus. The processor calibrates the machine data based on the calibration data and the attitude data.